1. Field of the Invention
The present invention relates generally to satellites and more particularly to satellite stationkeeping and momentum dumping.
2. Description of the Related Art
Satellites are placed in earth orbits to facilitate an ever-increasing number of operational functions (e.g., communications, weather monitoring and scientific observations). Accordingly, they are maintained in a variety of attitudes and placed in a variety of orbits (e.g., low earth orbits, transfer orbits, inclined synchronous orbits and geostationary orbits).
Synchronous satellites are typically required to remain within an assigned orbital station or "box" which has predetermined box dimensions (e.g., .about.147 kilometers). The act of maintaining a synchronous satellite in its assigned station is typically referred to as stationkeeping and it requires careful control of the satellite's longitude, the eccentricity of its orbit and the inclination of its orbital plane from the Earth's equatorial plane.
Perturbations which act to disturb a satellite's position include solar radiation pressure, gravity gradients due to the positions of the moon and the sun and gravity gradients caused by the earth's "triaxiality". These orbital perturbations must be offset by inducing controlled velocity changes in the satellite (e.g., velocity changes normal to the satellite's orbit plane and velocity changes tangential and radial to the satellite's orbit path). The velocity changes are typically effected with thrusters.
Specific attitudes of body-stabilized satellites must also be maintained to facilitate their designed functions (e.g., orientation stability is necessary for the antennas of communication satellites). A variety of external sources (e.g., solar pressure, gravity gradients and magnetic fields) can generate attitude-disturbing torques in satellites. These disturbance torques are typically countered with correction torques that are generated by changing the angular velocity of reaction wheel rotors. Because reaction wheels have an angular velocity design limit, however, their stored momentum must be periodically "dumped" by applying momentum-dumping torques to the satellite. These torques are typically effected with thrusters that are directed so that their forces define a moment arm with the satellite's center of mass.
Various satellite thruster systems and methods have been proposed. For example, U.S. Pat. No. 4,767,084 provides twelve thrusters which are arranged about a satellite's periphery. Six thrusters generate positive and negative torque about each of three axes of the satellite's coordinate system and the other six are backup thrusters. Two thrusters are mounted on each of the satellite's north and south faces and four thrusters are mounted on each of the satellite's east and west faces.
Initially, thrusters are repetitively fired on a selected one of the satellite's east and west faces to reduce the satellite's east-west drift. In these initial firings, thrusters are selected which also generate torque that simultaneously desaturates the momentum wheels. After the east-west drift has been reduced, thrusters are fired alternately on the east and west faces to complete the desaturation of the momentum wheels without disturbing the previously-reduced east-west drift.
A simpler thruster system was described in U.S. Pat. No. 5,443,231 which issued Aug. 22, 1995 to Bernard Anzel and was assigned to Hughes Electronics, the assignee of the present invention. This system positioned four thrusters on a satellite's antinadir face with their thrust lines passing through the satellite's center of mass. The thrust lines of north and south thruster pairs form a cant angle .theta. with the satellite's north-south axis and the thrust lines of east and west thruster pairs form a slew angle .alpha. about the satellite's north-south axis. Therefore, each thruster can generate normal, tangential and radial velocity changes in the satellite so that the thruster system is especially suited for use in methods of stationkeeping and momentum dumping.
In an exemplary use of this thruster system, copending U.S. patent application Ser. No. 08/701,513 (entitled "Stationkeeping and Momentum-Dumping Thruster Systems and Methods", filed Aug. 22, 1996, and assigned to Hughes Electronics, the assignee of the present invention) teaches methods for stationkeeping in inclined orbits.
In another exemplary use of this thruster system, copending U.S. patent application Ser. No. 08/841,211 (entitled "Momentum Unloading Using Gimballed Thrusters", filed Apr. 29, 1997, and assigned to Hughes Electronics) teaches methods for east-west and north-south stationkeeping while simultaneously unloading satellite momentum. In a method embodiment, thrusters are fired on each side of ascending and descending nodes (the intersections of the satellite's orbit plane and the earth's equatorial plane).
Although methods of these references can achieve satellite stationkeeping and momentum dumping, they fail to address the role of thruster fuel efficiency in these activities. Fuel efficiency is critical to the economic viability of satellites because increased efficiency facilitates greater payloads and longer lifetimes.